Working Your Body's Fuel Switches

I recently received a very interesting voice mail from Adam, one of my current group of "lab wabbits." He said he'd lost over 20 pounds of body fat between November and July, dropping from 17% to 12% bodyfat while adding over 4 pounds of lean mass.

Skeptics among you are likely thinking, "Yeah, big deal. It's called the newbie effect." Well, keep in mind that Adam has been training hard for over 10 years and is a world record holder in grip events, so he's about the furthest thing from a gym green horn as you're going to find.

Now add to the equation that Adam was reportedly consuming plenty of "evil" carbs, red meat, and beer as he achieved these results – and that his results were about average in the group of test wabbits.

Now have I got your attention?

Metabolic Flexibility: What the Heck Is It?

Metabolic flexibility is the ability to switch from one fuel source to the next. Due to possible discontinuities in both the supply and demand for energy, humans need a clear capacity to use lipid and carbohydrate fuels and transition between them.(1) This capacity is a healthy state called metabolic flexibility.

Theoretically, we could add in the intermediates like lactate but for the sake of simplicity we'll stick with the top 3.

Just so you can sleep at night, the use of protein as a fuel source is very limited unless in extreme conditions involving no exercise and very low amounts of dietary protein. As a result, we're down to two main fuel sources, fats (lipids) and carbohydrates.

Fuel source: Fat ←→ Carbs

Metabolic flexibility is the ability to switch from one fuel source to the next; from fats to carbs and carbs to fat. Metabolic inflexibility is just the opposite; the inability (or limited ability) to switch from one fuel source to the other. For a physique minded individual, this is a real pickle as it's awful hard to drop body fat if you can't burn it!

It's hypothesized that metabolic inflexibility may also play a role in various disease processes such as metabolic syndrome.(2,3,4,5) So along with better body composition, being metabolically flexible may even be healthy. Win-win!

Insulin: The Fuel Selector

A key to understanding metabolic flexibility is the vital role the storage hormone insulin plays. In a healthy state with normal insulin metabolism, humans can effectively switch from a primarily fat metabolism to a carbohydrate metabolism, and vice versa.

The great news for our "large and in charge" muscular friends is that skeletal muscle is a major player in energy balance due to its metabolic activity, storage capacity for both glycogen and lipids, and effects on insulin sensitivity.(7-9) Having more muscle allows your body to have a larger "drain" for both fats and carbs, since muscle is very metabolically demanding.

Victory Conditions

To further wrap our heads around this concept, we need to understand two primary conditions: fed and fasting.

During fasting conditions, someone who's very metabolically flexible will be able to tap into stored body fat. The cool thing is that individuals who are metabolically inflexible can also do this, so fasting becomes a way to increase metabolic flexibility.

During fed conditions, insulin levels will be much higher due to incoming energy (food). While a whole primer on insulin is beyond the scope of this article, the following (highly simplified) points are critical:

All foods result in the release of insulin.Insulin is the fuel selector switch.
When levels of insulin are low, the body is in fat burning mode.
When insulin is high, the body is shifted towards carbohydrate metabolism (and fat storing mode).

Kelley et al. presented data from subjects with type 2 diabetes showing them to be as metabolically inflexible as obese subjects, and they derived relatively less energy from lipid oxidation (fat burning) during resting conditions.(10) Our lean friends showed a greater suppression of lipid oxidation during insulin-stimulated conditions (increased insulin shifted them from fat metabolism to carb metabolism, a good thing).

Assessment of Metabolic Inflexibility

One way to assess metabolic flexibility is by the infusion of drugs (insulin, glucose, etc.) to alter the metabolic environment. The downside to this is it's difficult to perform even in a clinical setting, requires more specialized training, and is highly invasive. Considering that injecting insulin can have huge side effects like, uh, death and it becomes clear that a more practical method is required.

Condition 1: High Insulin

Taking in a whey protein drink with some simple carbs on a relatively empty stomach is an effective way to jack insulin levels up. I recommend people try this as their breakfast a few times during the week and monitor their reaction.

If you feel like collapsing face-down in your drink and spilling it all over the table, you probably have an issue with high levels of insulin and the resulting crash from it.
If you feel great, no worries.

Condition 2: Low Insulin

For this test, I recommend you do some fasting. Not the old school maple syrup, lemon juice, and cayenne pepper fasting, just no consumption of any calorie-containing foods.

Despite what you might've read in the weird wild web, any intake of food will result in the release of insulin, and the amount released is dependent on many things. Add to the mix the fact that everyone will respond differently and it's a doozy of a mess to sort out. The actual insulin data from subjects is all across the map.

In a fasting condition, there's no food coming in to stimulate any increase insulin. If you find that you can't make it more than two hours, you're probably inflexible to burning body fat. Not a good state to be in, my large friend. If you can easily go for 5-6 hours, that's much better.

I have advanced clients hit a 24-hour fast since there's data showing insulin levels reach their lowest point at around the 24 hour mark. Don't worry about the dastardly effects of catabolism; the amount of catabolic breakdown of muscle proteins is insignificant during a fast and isn't worth worrying about and releasing more cortisol.

If you're really concerned, just include one strength training session since the NET result of a strength session is an anabolic response.

Practical Application

If you're very metabolically flexible, eating some "bad" foods from time to time (or often as in this case) shouldn't be an issue, as your body can convert them into fuel without many side effects.

Try simulating the conditions for high and low levels of insulin and note your response. If you feel like a puddle after a protein and carb drink, you're more intolerant to high levels of insulin. This means it's time to add in some more low intensity cardio exercise or even a fast as fasting has been shown to improve insulin sensitivity.

If you can't make it more than two hours without some food, you're impairing your body's ability to burn that spare tire, so work to slowly increase your spacing between meals. I like to have clients do about one 24 hour fast per week if their goal is to drop fat and increase their metabolic flexibility.

Much of this throws what we think we know on its head, but I encourage you to experiment. You just might be pleasantly surprised!

Great article! and more good evidence leaning towards Intermittent fasting. I really have to start this after my skiing vacation to drop some pounds. This has been one of my favourite articles yet! I hope this reads okay, i just had a whole lot of kava.

I would PAY for an answer to these questions: I am a type 1 diabetic keep in mind... Im going to ask these questions as simply put as I can. I have had enough searching for answers and hope you all can help! 1. What does insulin sensativity mean? 2. Is being more insulin sensative good or bad? 3. If being insulin sensative is good, how so? I thought insulin is anabolic? So, the more insulin you produce/inject the more muscle mass you would obtain correct? 4. I am type 1 diabetic, and have been taking less and less insulin as I have been using IGF LR3 and my needs have gone down...AGAIN, is this good or am I setting myself up for LESS muscle gain by not injecting as much insulin?

"Insulin sensitivity is a term used to describe people who require relatively normal or low levels of insulin to process glucose. People with insulin resistance, on the other hand, need a lot of insulin to process glucose, and this leads to health problems. Several diagnostic tests can be used to determine how sensitive someone is to insulin, and these tests may be ordered if a doctor suspects that a patient is having difficulty with glucose metabolism.

The pancreas is responsible for secreting insulin. Insulin triggers tissues in the body to absorb glucose from the blood, lowering blood sugar levels so that they will remain relatively stable. These tissues can store glucose in the form of glycogen. In someone with insulin sensitivity, the insulin works as it should; when insulin-sensitive tissues like the liver and the muscles are exposed to the hormone, they respond by absorbing glucose.

In someone with insulin resistance, large amounts of insulin are needed to achieve the same effect. "

Thanks for all the kind words guys. Much appreciated. Let me know how I can help. As to the questions, this is just general advice since you will need to talk to your doc about your particular case as misuse of insulin can kill you. The above is great for a definition.2) more sensitive, in general, is better. BUT it varies depending on the tissue. In a perfect world, we want muscle to be sensitive to insulin and fat not to be as sensitive. Again, this is unfortunately and over simplification too. Insulin is anabolic in the sense that it cause STORAGE (fat or muscle). As for muscle, it appear to be mostly anti-catabolic3) we also need to keep in mind 1) the amount of insulin released and 2) how sensitive the receptors are in the body. The body can regulate specific TISSUE effects by altering the receptor, yet still releasing the same amount of insulin. 4) it would be determined by your current results. If your results are great, then why change?Hope that helps a bit and stay safe!Mike T Nelson

Hi Mike, I've got a related question that I've wondered about for a while. When for fat loss, growth hormone, etc insulin is talked about as a detractor, but I can't recall seeing any studies on those where it was insulin independent of nutrition. I've always wondered how sure the researchers were that it was truly the insulin level as the cause, and not the heightened glucose/amino/fatty acid levels (which of course cause heightened insulin levels). I guess I'm wondering how possible it is that there is some confusion between correlation and causation as far as insulin itself goes.

The short answer, is I would agree. Anytime we look at causation, it gets hard. Do we really ever know the cause? We do know how close events are to each other via association. I like association instead of causation, but I understand your point. The hard part is it's not ONLY insulin and the tissue sensitivity to insulin can vary a lot AND be different in different tissue (liver, vs muscle, vs fat are the big ones). If you have a particular study. I may be able to give you a better answer. If we look at Type 2 Diabetics, they can still lose weight, even with goofy insulin/glucose values; but it probably won't be as easy as someone who is more healthy. Rarely, is it ever ONE thing 100% of the time in physiology. The key is to try to narrow it down to the things that 1) matter 2) we have some control over; and for that, endogenous insulin fits nicely. Hope that helps! Rock on Mike T Nelson PhD(c)